1. Field of the Invention
The present invention generally relates to a zoom lens system, and more particularly to a wide angle zoom lens system.
2. Description of the Prior Art
In this field of art, there has been known a zoom lens system, which has a first negative lens group located at the front of the zoom lens system and a second positive lens group located at the image side of the first lens group with a variable air space formed between the first and second lens groups. In the zoom lens system, the first and second lens groups are shiftable along the optical axis with the variable air space changing during the zooming operation, and the first lens group alone is shiftable along the optical axis in the focusing operation. Such a zoom lens system is of an inverted telephoto type because of the negative front lens group, and is favorable for designing a wide angle lens system.
The above mentioned zoom lens system of an inverted telephoto type, however, is unfavorable when focusing on a close object, i.e. the field curvature of the lens system changes in response to the close focusing. Further, various additional changes in aberrations are caused by the movement of the first lens group in the focusing operation. In other words, the astigmatic difference is increased with the field curvature positively deviated, and the spherical aberration is negatively deviated, if the zoom lens system is focused to a close object by means of the shifting of the first lens group. U.S. Pat. No. 4,099,846 is cited of interest in zoom lens systems.